Who are the research subjects of Dr. Stone’s study?

W: Dr. Stone, you have studied 25 people who never get sick. Could you give us some easy-to-follow tips on how they’ve successfully stayed away from illness in their whole lives?
M: Our study shows that some habits will help you to stay well and live longer. Such as, cold shower, napping, weightlifting and swimming.
W: Do you mean that taking a cold shower is good to our health?
M: Yes. Professor Smith is 82 years old now. He has taken a cold shower every day for most of his adult life, and never suffers from colds.
W: That sounds great. Maybe I can try it myself. What else can we do to keep us against illness?
M: Besides exercise, friends are also very important. You might not think having a social network would count as much as diet and exercise when it comes to good health. But studies have found that good friends can protect against everything from memory-lost to heart attacks, to infectious diseases. Mary is a former nurse. She can’t remember being ill in all her 75 years, and she credits this to the fact that she is constantly seeking out new friends. Sociability increases self-respect which in turn supports the immune system.